Bug ID 483953: Cached route MTUs may be set to the value of TM.MinPathMTU even if the path MTU is lower than that value.

Last Modified: Apr 10, 2019

Bug Tracker

Affected Product:  See more info
BIG-IP LTM(all modules)

Known Affected Versions:
10.2.4, 11.2.1, 11.3.0, 11.4.0, 11.4.1, 11.5.0, 11.5.1, 11.5.1 HF1, 11.5.1 HF10, 11.5.1 HF11, 11.5.1 HF2, 11.5.1 HF3, 11.5.1 HF4, 11.5.1 HF5, 11.5.1 HF6, 11.5.1 HF7, 11.5.1 HF8, 11.5.1 HF9, 11.5.2, 11.5.2 HF1, 11.5.3, 11.5.3 HF1, 11.5.3 HF2, 11.5.4, 11.5.4 HF1, 11.5.4 HF2, 11.5.4 HF3, 11.5.4 HF4, 11.5.5, 11.5.6, 11.5.7, 11.5.8, 11.5.9, 11.6.0, 11.6.0 HF1, 11.6.0 HF2, 11.6.0 HF3, 11.6.0 HF4, 11.6.0 HF5, 11.6.0 HF6, 11.6.0 HF7, 11.6.0 HF8, 11.6.1, 11.6.1 HF1, 11.6.1 HF2, 11.6.2, 11.6.2 HF1, 11.6.3, 11.6.3.1, 11.6.3.2, 11.6.3.3, 11.6.3.4, 11.6.4, 12.0.0, 12.0.0 HF1, 12.0.0 HF2, 12.0.0 HF3, 12.0.0 HF4, 12.1.0, 12.1.0 HF1, 12.1.0 HF2, 12.1.1, 12.1.1 HF1, 12.1.1 HF2

Fixed In:
13.0.0, 12.1.2

Opened: Oct 10, 2014
Severity: 3-Major
Related AskF5 Article:
K55042294

Symptoms

ICMP type 3 code 4 (needsfrag) messages are elicited when TMM transmits packets at the TM.MinPathMTU size if the path MTU is lower than that value.

Impact

TMM may use and enforce a low path MTU for clients capable of handling a higher path MTU, but may use an MTU too high to reach clients whose path MTU is lower than TM.MinPathMTU. This metric will live for 10 minutes by default.

Conditions

Path MTU discovery results are cached by default. If a client responds to an IP datagram with an ICMP needsfrag message with a very small MTU (smaller than the value of the TM.MinPathMTU database variable), the cached path MTU value will be set to the TM.MinPathMTU value even though this still isn't able to traverse the path. This can affect multiple endpoints when a low MTU is advertised by an endpoint (misconfigured or malicious) behind a shared NAT address.

Workaround

This issue has no workaround at this time. The route metric lifetime can be lowered using route.metrics.timeout db key.

Fix Information

Path MTUs lower than the value of TM.MinPathMTU will no longer be cached by TMM.

Behavior Change